I don't mind if it's just a page or two - as someone else said, some of the retro ads can be quite entertaining, as can suggestions on what to read next can be quite entertaining, and they're part of a long-standing tradition in print books. I'm not that keen on these being just a long list of that particular author's books - I prefer to see a selection of different synopses. OTOH, they should be
synopses - Elfwreck's quite right about the page count. I absolutely
loathe 
getting to the end of a book and finding a great chunk of it is a chapter from the author's next one. I never read them and it does make me feel duped, and TBH, it also make me wonder how good a book is likely to be if the author's managed to churn it out in the time taken to publish the previous one - either way, it's not calculated to encourage me to buy!